Currently, semiconductor single crystals such as those composed of silicon are produced by the CZ method in many cases. As shown in FIG. 3, an apparatus for producing a semiconductor single crystal used for this CZ method comprises a crucible 4 in which a raw material 12 is charged, a heater 2 surrounding the crucible 4, a heat insulating material 8 disposed around the heater, pulling means for bringing a seed crystal 14 into contact with a melt in the crucible and growing a single crystal, and a metal chamber 17 for accommodating the aforementioned members.
In a conventional growing process of a semiconductor single crystal by the CZ method, a single crystal 13 is grown by using the aforementioned apparatus for producing a semiconductor single crystal, in which process the raw material 12 is charged in the crucible 4 and heated by the heater 2 surrounding the crucible 4 to form a melt, and the seed crystal 14 is brought into contact with the melt and slowly pulled with rotation to grow the single crystal 13. The single crystal 13 is pulled so that it should have a portion having an increasing diameter 13a, a portion having a constant diameter (also referred to as “straight body”) 13b, which can be used as a product, and a portion having a decreasing diameter 13c, and it is detached from the melt after the portion having a decreasing diameter 13c is formed.
In order to obtain a high yield in the production of a single crystal, it is necessary to form the portion having a constant diameter as long as possible in contrast to the lengths of the portion having an increasing diameter and the portion having a decreasing diameter, and it is necessary that the pulling is performed from the melt in an amount as large as possible. If the diameter of the crystal becomes larger, the proportions of the portion having an increasing diameter and the portion having a decreasing diameter become larger. Therefore, the aforementioned necessity becomes more important, and it becomes more necessary to use a larger amount of the raw material by using a crucible having a larger diameter.
Further, as another factor for obtaining a high yield, it is also important to pull a single crystal ingot until the remaining amount of the melt reaches the possible smallest amount. In this specification, a value representing a ratio of weight of a grown single crystal relative to weight of initial raw material before the crystal is pulled, which is expressed in terms of percentage, is called a single crystal formation ratio. If this term is used, it can be said that, in order to pull a crystal having a large diameter with a high yield, it is important to use a crucible having a large diameter and, at the same time, pull a single crystal ingot at a high single crystal formation ratio.
However, since the radiant heat quantity from the melt surface increases with use of a crucible having a larger diameter, heating from the lateral direction only by the heater surrounding the crucible tends to become insufficient in terms of heating quantity. In particular, in the period of pulling of the latter half of the straight body of single crystal ingot or after the single crystal ingot is detached, the heat receiving area from the lateral direction is decreased, because the melt becomes shallow, and therefore the heating efficiency is decreased. As a result, a phenomenon of solidification of the melt frequently occurs. The term “solidification” used herein means a phenomenon that the surface or a part of internal portion of the melt or the whole melt is cooled and becomes solid. The worst result due to the solidification phenomenon is caused when a solid portion formed in the melt in contacted with the growing interface of the crystal. In such a case, because the crystal grows with dislocations thereafter, the pulling of the single crystal becomes, impossible. Further, when most of the melt is solidified, a stress is applied to a crucible due to the volume change of the melt (volume expansion in the case of silicon) and cracks may be generated in the crucible. Therefore, it may be necessary to stop the pulling of the crystal thereafter.
In order to avoid such solidification of the melt, it is contemplated to increase the electric power supplied to the heater surrounding the crucible as shown in FIG. 4. However, if the heating quantity supplied from the heater is increased, the temperature gradient of the single crystal under pulling becomes small, and thus the growing rate of the single crystal must be lowered. Alternatively or moreover, the crystal is gradually cooled in the temperature region in which crystal defects are formed, and thus there is caused a problem that defective size becomes larger. The crystal defects formed in this case are cavities called voids or aggregations (clusters) of dislocations, which are formed as a result that vacancies or interstitial atoms incorporated at the growing interface of the crystal aggregate during the subsequent cooling.
Further, if the electric power supplied to the heater is unduly increased, the temperature of the crucible is excessively elevated. As a result, the crucible may be deformed, or the internal surface of the crucible contacting with the melt may be degenerated. If the crucible is significantly deformed, the crucible touches the single crystal and the members including the heater, and it becomes unavoidable to interrupt the pulling. Further, when the crucible surface that is in contact with the melt is degenerated, the degenerated portions may fall off into the melt and reach the glowing interface of the crystal, so that dislocations may be generated in the single crystal.
While the problems to be caused during the pulling of crystal when the amount of the melt remained in the crucible becomes small have been explained above, the problems concerning the solidification and degradation of crucible due to excessive heating by heater are similarly observed with respect to the melt remained in the crucible after a single crystal is detached. That is, the solidification of the melt remained in the crucible poses a problem when multi-pulling is performed. The term “multi-pulling” used herein means that, after a grown single crystal ingot is detached from a melt and taken out from an apparatus for producing a single crystal, a raw material is additionally introduced into the melt remained in a crucible and melted and then a seed crystal is brought into contact with the melt to pull another single crystal ingot again.
In the multi-pulling, it is necessary to prevent the melt from rapidly solidifying during the period after a single crystal is detached and before finishing the additional charge of the raw material and melting of the whole raw material in the crucible. In particular, when a raw material is newly added to a small amount of the melt, heat of the melt is taken by the introduced raw material and the temperature of the melt may be rapidly decreased. At that time, if the melt is rapidly solidified and thus an abnormal stress is applied to the crucible due to the volume change of the raw material, cracks may be generated in the crucible.
As explained above, conventional techniques could not prevent the solidification of melt in a crucible without effecting crystal quality and operability when a crystal is pulled at a high single crystal formation ratio by using a crucible having a large diameter, or when a raw material is additionally introduced into a limited amount of melt in the multi-pulling, and therefore suitable means for solving the problems have been desired.